1. Field of the Invention
The present invention relates to a CCD-type solid-state pickup device and its fabrication method capable of decreasing the number of smear signals to be generated.
2. Description of the Prior Art
In the case of a CCD (Charge Coupled Device)-type solid-state pickup device, when a high-luminance object is present in a pickup screen, a false signal referred to as smear may be generated in the region.
FIG. 1 is a sectional view showing the structure of a conventional solid-state pickup device. Hereafter, this is referred to as the first prior art. A P-well layer 2 is formed at the surface of an N-type semiconductor substrate 1. An N-type impurity layer 3 serving as a light-receiving section is selectively formed at the surface of the P-well layer 2. Moreover, an N-well layer 4 serving as an electric-charge transfer section is formed at the surface of the P-well layer 2 in the region where the N-type impurity layer 3 is not formed. A P.sup.+ impurity layer 5 for isolating a device is formed at the both sides of a set of N-well layer 4 and N-type impurity layer 3.
Moreover, a gate insulating film 6 is selectively formed on the substrate 1 in the region where the N-type impurity layer 3 is not formed, and a polycrystalline silicon gate electrode 7 is formed on the gate insulating film 6. A silicon oxide film 8 is formed on the upside and the side-wall of the polycrystalline silicon gate electrode 7 and the surface of the N-type impurity layer 3. A CVD oxide film 9 is formed on the entire surface of them. A light shielding film 10 made of aluminum or the like is formed on the CVD film 9 except some regions above the N-type impurity layer 3. Moreover, a surface protective film 11 made of a CVD insulating film is formed on the entire surface of them.
In the case of a solid-state pickup device thus constituted, particularly a diagonally-incoming incident ray 12 among the-rays coming into the N-type impurity layer 3 serving as a light receiving section reflects on the surface of the N-type impurity layer 3 and the interface between the light shielding film 10 and the CVD oxide film 9. Then, the incident ray 12 is introduced into the N-well layer 4 serving as an electric-charge transfer section. As a result, the introduced incident ray is photoelectrically transferred to an electric charge and the electric charge is added to an original signal as a false signal. Because the false signal is superimposed on every signal electric-charge group in the N-well layer 4, a linear smear signal appears on a screen.
Therefore, a solid-state pickup device for decreasing the generation of the smear signal is disclosed in Japanese Examined Patent Publication (Koukai) No. Heisei 7-161957. Hereafter, this is referred to as the second conventional example. FIGS. 2A and 2B are sectional views showing a method for fabricating the solid-state pickup device of the second prior art in order of fabrication step. First, the structure of the solid-state pickup device of the second prior art is described below by referring to FIG. 2B. In the second prior art shown in FIG. 2B, a component same as that shown in FIG. 1 is provided with the same symbol and its detailed description is omitted.
As shown in FIG. 2B, in the case of the second prior art, a first protective film 13a made of a silicon nitride film is formed on the upside of the polycrystalline silicon gate electrode 7. Moreover, a second protective film 13b made of a silicon nitride film is formed on the side wall surface of the polycrystalline silicon gate electrode 7.
Then, a method for fabricating the solid-state pickup device of the second prior art is described below. As shown in FIG. 2A, the P-well layer 2 is first formed at the surface of the N-type semiconductor substrate 1. Then, the N-type impurity layer 3, N-well layer 4, and P.sup.+ impurity layer 5 are selectively formed at the surface of the P-well layer 2. Then, an insulating film (not shown), a polycrystalline silicon film (not shown), and a first silicon nitride film (not shown) are formed on the entire surface in order. Thereafter, the first protective film 13a, polycrystalline silicon gate electrode 7, and gate insulating film 6 are formed by using a resist film patterned into a predetermined shape as a mask and thereby, etching the first silicon nitride film, polycrystalline silicon film, and insulating film. Then, the silicon oxide film 8 is formed by thermally oxidizing the surface of the N-type impurity layer 3 at the surface of the semiconductor substrate 1. The silicon oxide film 8 has a thickness of, for example, 80 nm.
Thereafter, as shown in FIG. 2B, the second protective film 13b is formed on side wall surfaces of the polycrystalline silicon gate electrode 7 and the protective film 13a by forming a second silicon nitride film (not shown) on the entire surface of them and anisotropically etching the film. Thereafter, the CVD oxide film 9 for insulating the polycrystalline silicon gate electrode 7 from a light shielding film to be formed later is formed at a thickness of, for example, 150 nm on the entire surface of them. Thereafter, the light shielding film 10 is formed on the CVD oxide film 9 except some regions above the N-type impurity layer 3. Thereafter, the surface protective film 11 is formed on the entire surface of them.
In the case of a solid-state pickup device thus constituted, the top and side wall surfaces of the gate electrode 7 are covered with the protective films 13a and 13b respectively made of a silicon nitride film having a function of absorbing light. Therefore, even if the incident ray 12 diagonally coming into the N-type impurity layer 3 reflects on the surface of the N-type impurity layer 3 and the interface between the light shielding film 10 and the CVD oxide film 9 and then, reaches the protective films 13a or 13b, the incident ray 12 is absorbed by these protective films. Therefore, it is possible to prevent the light from introducing into the N-well layer 4 serving as an electric-charge transfer section and thereby, decrease the number of smear signals to be generated.
However, a CCD-type solid-state pickup device may have a transfer electrode made of polycrystalline silicon of at least two layers. Therefore, when fabricating a solid-state pickup device having a transfer electrode of two layers or more by the above fabrication method, problems occur that the number of fabrication steps increases and moreover, the fabrication cost increases. That is, the number of fabrication steps increases because it is necessary to perform the step of forming a silicon nitride film twice and the step of forming the protective film 13b by etching back a second silicon nitride film once whenever forming a one-layer transfer electrode.
Moreover, because a silicon nitride film has a function of absorbing light, a ray coming into a light-receiving section may be absorbed by the silicon nitride film if the silicon nitride film remains on a silicon oxide film on the N-type impurity layer 3. Therefore, to form a transfer electrode of two layers or more, a first-layer transfer electrode is formed and then, a silicon nitride film is formed on the entire surface and a protective film is formed on the side wall surface of a transfer electrode by etching back the silicon nitride film. In this case, it is necessary to completely remove the silicon nitride film from the surface of the light-receiving section. Therefore, to form a protective film on the side wall surface of the first-layer transfer electrode, it is necessary to over-etch the silicon nitride film formed above the N-type impurity layer 3. However, the thickness of a gate insulating film located under the second-layer transfer electrode is easily fluctuated due to the fluctuation in the etching rate of the silicon nitride film. As a result, the channel potential becomes unstable and a trouble occurs when electric charges are transferred.